Pigment epithelium derived factor (PEDF) was originally identified in conditioned medium of fetal human retinal pigment epithelium cell cultures. It shares sequence and structure homology to members of the superfamily of serine protease inhibitors (serpin), however, it does not serve as an inhibitor of any known protease activity.
PEDF was first described as a neurotrophic factor that induces a specific neuronal phenotype in retinoblastoma cells (Steel, F. R. et al. Proc. Natl. Acad. Sci. U.S.A. 90: 1526-1530, 1993). The neurotrophic activity of PEDF was also demonstrated by its ability to support neuronal survival (Taniwaki, T. et al. J. Neurochem. 64: 2509-2517, 1995), and its ability to protect neurons against neurotoxic effects. Structure-function studies have shown that this neurotrophic activity is exerted by the amino terminal segment (44-mer, amino acid residues 78-121) of the human PEDF, and that its activity is mediated through a ˜80 kDa membranal receptor, which is abundant in retinoblastoma cells, and in neural retinal cells.
Besides its neurotrophic activity, PEDF was further demonstrated to be one of the most potent natural inhibitors of angiogenesis (Dawson, D. W. et al. Science 285: 245-248, 1999). Thus, it was found that PEDF inhibits not only bFGF-induced migration of endothelial cells under in vitro conditions, but also bFGF-induced neovascularization in an avascular rat cornea. Furthermore, addition of anti-PEDF antibodies (Abs) to rat corneas was found to stimulate the invasion of new vessels into these corneas, suggesting that PEDF plays a physiological regulatory role in retinal angiogenesis. PEDF was also shown to be a very potent inhibitor of neovascularization in a murine model of ischemia-induced retinopathy (Stellmach, V. V. et al. Proc. Natl. Acad. Sci. U.S.A. 98: 2593-2597, 2001). The anti-angiogenic activity of PEDF was associated with endothelial cell apoptosis, probably by increasing Fas ligand (FasL) mRNA and surface FasL in these cells.
It was recently reported that besides its expression in multiple sites in the eye, PEDF is also present in human plasma, at a physiologically relevant concentration (Petersen, S. V. et al. Biochem. J. 374: 199-266, 2003). In the last decade several reports have described the possibility that protein kinases might function as a regulatory device not only intracellularly, but also in the cell exterior (Redegeld, F. A. et al. Trends Pharmacol. Sci. 20: 453-459, 1999). These reports described the presence of membrane-bound ectoprotein kinases (on the outer cell surface) and soluble secreted exoprotein kinases (detached from the cell). Additionally, it was shown that these ecto- or exoprotein kinases do have several substrates in the circulating blood including the coagulation cofactors Va and VIII as well as vitronectin. The main protein kinases that seem to exert exokinase activity are protein kinase A (PKA) and protein kinase CK2 (CK2). For example it was shown that vitronectin is phosphorylated by PKA and this phosphorylation modulate its interaction with PAI-1. In addition, phosphorylation by CK2 changes intracellular signaling by vitronectin, indicating that both PKA and CK2 play an important regulatory role in the circulating blood.
U.S. Pat. No. 5,840,686 to Chader et al., discloses nucleic acids that encode PEDF and a truncated PEDF, the equivalent proteins, and methods for producing recombinant PEDF and the truncated PEDF. U.S. Pat. No. 5,840,686 claims a method of prolonging neuronal cell survival and a method for inhibiting glial cell proliferation comprising administering recombinant PEDF. U.S. Pat. No. 6,319,687 to Chader et al., claims a recombinant PEDF protein (418 amino acids) and truncated forms of PEDF having neurotrophic as well as gliastatic activity.
PCT Application WO 01/58494 claims a method of treating an ocular-related disease in an animal. The method comprises expression of an angiogenesis inhibitor and a neurotrophic agent in an ocular cell using an expression vector that contains the nucleotide sequence for these factors. A preferred angiogenesis inhibitor is PEDF, which is known to exert both anti-angiogenic and neurotrophic activities.
U.S. Pat. No. 6,451,763 to Tombran-Tink et al., discloses the purification of PEDF from culture medium of human retinal pigment epithelial cells and claims methods of treating retinal diseases comprising administering PEDF to subjects in need thereof. It is also disclosed that in addition to retinal pigment epithelial cells, PEDF may be isolated from the vitreous humor of human, bovine, monkey and other primates. Since PEDF is abundant in the vitreous humor and since the vitreous humor is easily removed from the eyecup, the vitreous humor was suggested to be the easiest source from which PEDF can be isolated.
U.S. Pat. No. 6,797,691 to Bouck et al., discloses methods of inducing differentiation and slowing the growth of a neuroblastoma cell comprising administering PEDF to the cell.
International Patent Application WO 03/059248 discloses that PEDF is present in human plasma at physiologically relevant concentrations and exhibits potent anti-angiogenic and neurotrophic activities.
The inventors of the present application have shown that mutations of the phosphorylation sites of PEDF affected its anti-angiogenic and neurotrophic activities (Seger et al., The Weizmann Institute BioScience Open Day May 17, 2004; Seger et al., FEBS Lecture Course on Cellular Signaling, May 21-27, 2004; Seger et al., Blood, in press, 2004), but have not disclosed the utility of the present variants.
It would be highly advantageous to have PEDF variants having greater selectivity in terms of their anti-angiogenic and neurotrophic activity than native or wild-type PEDF.